The present invention relates to measuring air pollutant emissions from combustion sources which burn sulfur containing materials (e.g. coal fired power plants). More particularly the invention relates to monitoring systems (and related methods) which can measure sulfur trioxide/sulfuric acid levels in such flue gas.
Power plants and many other facilities are usually required to keep emissions of exhaust gases below specified levels, and further often required to monitor and periodically report to appropriate regulatory agencies their performance in doing so. If the fuel they use contains sulfur (as is typical for coal and some oil-based fuels), sulfur oxides will be produced as a byproduct of the combustion. Of those sulfur oxides, approximately 0.7% is typically sulfur trioxide.
Further, other downstream pollutant removal devices, such as nitrous oxide control devices, may alter the relative amount of sulfur dioxide to sulfur trioxide. Also, if (as is typical) there is moisture/humidity in the exhaust stack, sulfur trioxide will convert in the stack to sulfuric acid.
Where the plant tries to avoid exceeding emissions limits by using low sulfur fuels (without special capture equipment), it is desirable to monitor the exhaust stack to confirm that the fuel in fact has sulfur levels that are sufficiently low so as to avoid the need for using sulfur oxide removal technologies. Where higher sulfur fuels are used, and sulfur oxide removal technologies are applied upstream of the exhaust stack, it is still desirable to periodically monitor the success of the removal technologies (e.g. to spot maintenance issues).
Sulfur trioxide is of particular concern because it is aggressively hygroscopic and will quickly form sulfuric acid in the presence of humidity. As is well known, releasing substantial amounts of sulfuric acid into the air can cause significant adverse impacts as a condensable particulate. Also, sulfuric acid can affect mercury control systems and corrode duct work and other equipment.
Hence, there have been extensive efforts to measure sulfur trioxide/sulfuric acid levels in flue gas. A standard method of measurement is to extract samples of gas from the flue, provide the sample with sufficient moisture to convert all sulfur trioxide in it to sulfuric acid, condense the sulfuric acid, and measure the quantity of sulfuric acid collected in a given extracted sample (e.g. by using ion chromatography). While this works reasonably well when measuring relatively high concentrations of sulfur trioxide/sulfuric acid, it is susceptible to significant bias when measuring relatively low concentrations. This is of increasing concern because permitted regulatory emission levels are dropping over time, increasing the need for accuracy at low concentration measurements.
The error/bias problem is compounded when the sample is a complex variable mix of flue gas with particulate. Clogging and/or premature condensing problems due to this can distort the accuracy of various known measurement equipment.
In U.S. Pat. No. 7,029,920 it was proposed to sample stack gas using a probe projecting into the stack. Rather than shutting the normal sampling off completely periodically for calibration, this patent instead taught, on a periodic basis, testing sample gas mixed with a known amount of spiked nitrogen oxide containing gas, and then using the resulting measurement to try to assess bias when measuring nitrogen oxides.
However, this system required one to assume that the flue gas contaminant content was essentially the same at the time of unspiked sampling as when spiked sampling was happening before or thereafter, and spiking was focused on nitrogen oxide gas measurements. Further, it relied on a readily available supply of nitrogen oxide based spiking gas of reliable concentration. Also, it required an external construction to accommodate the spiking gas source. This system is not readily adaptable for assessing sulfur trioxide/sulfuric acid measurement variability, and still had some unaccounted for biases.
One problem in applying this approach to sulfur trioxide/sulfuric acid testing is there exists no readily available source of sulfur trioxide gas with carefully controlled concentration values, and hence a reliable “known” is currently unavailable as a practical matter. One could generate sulfur trioxide from commercially available sources of sulfur dioxide, and then use that sulfur trioxide for calibration. However, techniques have not yet been reported for reliably producing a known supply with a sufficiently stable concentration of sulfur trioxide for this purpose.
Thus, a need exists for improvements in the equipment and methods used to monitor flue gas for small concentrations of sulfur trioxide/sulfuric acid.